Seeing a movement of the shadow is also dependent
on the surface on which you see the shadow. If
this surface is very even colored/structured it
will be more difficult to see a movement then
when the surface has such a structure (like fine
grain) that it is easy easier to see the movement..
Thibaud
At 10:40 7-8-2013, JOHN DAVIS wrote:
Hi Kevin (and Peter R et al),
I saw your SML post and will watch with interest what responses it gets.
One point, though: the value of 1.27 mm/sec as
the lower limit of perception of movement is
impossibly precise! I suggest that you would get
+/-50% variation between different observers.
Also, it will depend heavily on the situation:
watching a laser spot on a piece of graph paper
will give a value orders of magnitude different
to following a distant aircraft on a clear blue
sky. What really matters is the rate of angular
change and a stationary reference point.
Regards,
John
----------------------
Dr J Davis
Flowton Dials
From: Kevin Nute <[email protected]>
To: [email protected]; Peter Ransom
<[email protected]>; JOHN DAVIS <[email protected]>
Sent: Tuesday, 6 August 2013, 21:38
Subject: Visibly Moving Gnomon Shadows
The movement of the gnomon shadow at the famous
Samrat Yantra equitorial sundial in Jaipur is
reputed to be clearly visible to someone
standing near the projection surface. I've read
it moves as fast as 1 mm/s, though obviously not
all the time. At a given latitude, say 40ยบ N,
can anyone suggest a simple formula for
estimating how far a projection surface would
need to be from a vertical or horizontal gnomon
for the shadow to move at 1.27 mm/s (the
practical lower threshold of perceptible
movement) I wonder? Or in other words, what's
the smallest sundial you could build to see
real-time movement of the gnomon shadow with the naked eye?
Kevin Nute
Professor of Architecture
University of Oregon
School of Architecture and Allied Arts
Eugene, OR 97403
USA
<mailto:[email protected]>[email protected]
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